Cranial anatomy of the "round-headed" Amphisbaenian Zygaspis quadrifrons (Squamata, Amphisbaenia) based on high-resolution x-ray computed tomography.

Amphisbaenians are a poorly understood clade of fossorial lizards. Because of their derived anatomy and relative scarcity, the systematics of the clade and its placement within squamates has long been controversial. Traditional approaches grouped species into four assemblages according to burrowing behavior and cranial morphology, resulting in the recognition of "shovel-headed," "round-headed," "keel-headed," and "spade-headed" morphotypes. Recent phylogenetic analyses do not support the monophyly of the taxa that share those morphotypes. Detailed analyses of cranial osteology were previously accomplished using high-resolution x-ray computed tomography (HRXCT) for the "shovel-headed" Rhineura hatcherii (Rhineruidae) and the "spade-headed" Diplometopon zarudnyi (Trogonophidae). A detailed description of the "round-headed" Amphisbaena alba was previously completed based upon traditional "dry" skeletal specimens. Seven species of the "round-headed" Blanus (Blanidae) were also analyzed using HRXCT. The goal of that project was a comparative analysis of all extant species of Blanus rather than a detailed, bone-by-bone description of one species, but certainly is useful for comparison with another "round-headed" taxon. The "round-headed" morphotype is by far the most common among amphisbaenians and is much in need of further documentation. We use HRXCT imagery to provide additional data about the disparity in cranial morphology among amphisbaenians. Those data allow us to provide another detailed description of a "round-headed" amphisbaenian, the poorly known southern African species Zygaspis quadrifrons. HRXCT is ideal for this relatively rare and diminutive species. We are able to visualize and describe a detailed reconstruction of the entire skull as well as individual cranial elements. Comparisons with other species that were described in similar detail-D. zarudnyi, Spathorhynchus fossorium, R. hatcherii, and A. alba-and to a lesser degree with Blanus, reveal a complex mosaic of morphological features of the skull in Zygaspis. Preliminary data suggest that intraspecific variation is present within Z. quadrifrons, and interspecific variation among other species of Zygaspis may be sufficient for species-level recognition based on cranial osteology. Our description is, therefore, also intended to serve as a baseline for comparative analysis of other specimens of Z. quadrifrons and of other species within the genus.